80/40m Loaded Dipole

Most of the time Amateurs build loaded 80 m Dipoles based on a 40 m Dipole, i.e. design a 40 m Dipole and then load the Dipole to make the antenna resonant on 80 m. I have been asked why I haven't installed this antenna at my QTH. I will give my answer a little bit later, but first, we'll have a look at the antenna from the comfort of the living room (my shack is to cold).

Figure 1 shows the modelled dimensions of the antenna for best VSWR at 7 m height over poor ground. The two coils, L1, have an inductance of 120 µH and a Q of approx. 100. The basic design of the antenna is a ½λ Dipole on 40 m with approximately 22 m span. Adding the 80 m loading coils and pigtails will increase the overall length of the antenna to 26 m.  

Lets see how the 40 m part of the antenna would perform in a real installation. The design frequency for the 40 m ½λ Dipole is 7.1 MHz and the height of the antenna will be 7 m over poor ground, my QTH. Figure 2 shows a modelled VSWR.

The 2:1 VSWR bandwidth is more then 200 kHz and the 3:1 VSWR greater then the whole 40 m band. Most modern rigs with a build in Antenna Tuning Unit (ATU) will be able to handle this. And the old boat-anchors with their Collins-filter should have no problem to transfer all of their RF-power into this antenna. 

Figure 3 shows us the modelled gain of the dipole at 7 m above poor ground.

To check the performance, I've modelled a 40 m Dipole with the same specs, i.e. height, ground and wire specifications, and then compared the data.

Figure 4 shows the total gain in the vertical plane of both Dipoles.

As expected, there is not much differences between the antennas.

Figure's 5 and 6, are showing the expected impedance's of the respected antenna.

Figure 5 shows that the 80/40 m Dipole has an impedance of approx. 71Ω at resonance and,

Figure 6 shows the impedance of the 40 m Dipole to be approx. 68Ω at resonance.

The reason for the difference could be the end-loading of the 40 m element of the 80/40 m Dipole. Both antennas could be feed directly with 50Ω coaxial cable with acceptable VSWR,  however I would strongly recommend to use a 1:1 BALUN.

So what about the 80 m performance of this antenna. Well, lets recall that the antenna is rather short and has been loaded to be resonant at 3.6 MHz. A half wave at this frequency is about 40 m and our 80/40 m Dipole is only 26 m in length. So we have to expect that there will be limitations.

As can be seen from Figure 7, the 2:1 VSWR bandwidth is approx. 40 kHz and the 3:1 VSWR approx 60 kHz.

Next we have a look at the gain of the antenna at 80 m, Figure 8.

Figure 8 shows us a modelled gain of 0.5 dB at  7 m above ground, which is approx. 2 dB less then a full size Dipole.

And Figure 9 shows us an impedance curve of the 80 m part of the 80/40 m loaded Dipole, approx. 75Ω at resonance.

Figure 10 shows the performance difference of the two antennas, red the 80/40 m and blue a full size 80 m Dipole.

At Figure 10 we can see that we should expect at least a 2.5 dB difference between the 80 m dipole and the 80/40 m loaded Dipole in favour of the 80 m Dipole.

So to wrap it up, the antenna is a no compromise 40 m antenna and should work as good as a standard 40 m Dipole. On 80 m the performance is down by 2.5 dB and the usable bandwidth is a bit limited. However, with only a 26 m span the antenna will get you on air on 80 m and 40 m. As always, height plays a big role in antenna performance and element length. The given dimensions are for my installation, i.e. 7 m above poor ground. Other antenna installations will differ in element length and performance.

But with a span of only 26 m the antenna should fit into the smallest of a suburban block and get you on the Air on 40 m and 80 m.

Now, why didn't I install the antenna at my QTH?

Well, one of my requirements for 80 m was a 2:1 VSWR bandwidth of 100 kHz. Checkout the results of that quest here.